Color reproduction is typically measured using the Color Rendering Index (CRI Ra). CRI Ra is a modified average of the relative measurements of how the color rendition of an illumination system compares to that of a reference radiator when illuminating eight reference colors, i.e., it is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator. Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80).
Solid state light sources may be utilized to provide colored (e.g., non-white) or white LED light (e.g., perceived as being white or near-white), as has been investigated as potential replacements for white incandescent lamps. A solid state lighting device may include, for example, at least one organic or inorganic light emitting diode (“LED”) or a laser. A representative example of a white LED lamp includes a package of a blue LED chip, made of InGaN and/or GaN, coated with a phosphor (typically YAG:Ce or BOSE). Blue LEDs made from InGaN exhibit high efficiency (e.g., external quantum efficiency as high as 60%). In a blue LED/yellow phosphor lamp, the blue LED chip produces an emission with a wavelength of about 450 nm, and the phosphor produces yellow fluorescence with a peak wavelength of about 550 nm upon receipt of the blue emission. Part of the blue ray emitted from the blue LED chip passes through the phosphor, while another portion of the blue ray is absorbed by the phosphor, which becomes excited and emits a yellow ray. The viewer perceives an emitted mixture of blue and yellow light as white light. A blue LED and yellow phosphor device typically good efficacy but only medium CRI Ra (e.g., between 70 and 80), or very good CRI Ra and low efficacy.
Various methods exist to enhance cool white light to increase its warmth. To promote warm white colors, an orange phosphor or a mix of red and yellow phosphor can be used. Cool white emissions from a white emitter may also be supplemented with red and/or cyan LEDs, such as disclosed by U.S. Pat. No. 7,095,056 (Vitta) to provide warmer light.
As an alternative to stimulating a yellow phosphor with a blue LED, another method for generating white emissions involves combined use of red, green, and blue (“RGB”) light emitting diodes in a single package. The combined spectral output of the red, green, and blue emitters may be perceived by a user as white light. Each “pure color” red, green, and blue diode typically has a full-width half-maximum (FWHM) wavelength range of from about 15 nm to about 30 nm. Due to the narrow FWHM values of these LEDs (particularly the green and red LEDs), light made from combinations of red, green, and blue LEDs may exhibit low efficacy in general illumination applications.
Another example of a known white LED lamp includes a package including ultraviolet (UV) based LEDs combined with red, green, and blue phosphors. Such lamps provide acceptably high color rendering, but exhibit low efficacy due to Stokes losses.
It is known to mount solid state light sources, such as semiconductor light emitting devices in packages that may provide protection, color enhancement, focusing, and similar utilities for light emitted by a light emitting device. Examples of such packages are disclosed in U.S. Patent Application Publication Nos. 2005/0269587, 2004/0126913, and 2004/0079957.
Output efficiency and thermal management present ongoing concerns with known solid state emitter devices. Certain end uses such as grow lights to promote photosynthesis obtain limited benefit from solid state light sources intended for illumination pleasing to a human viewer. Solid state emitter packages as described in the above-referenced publications may be suitable for high power, solid state illumination applications; however, factors such as controllability, output efficiency, thermal management, and manufacturability present ongoing concerns. There remains a need for improved packages each including multiple LEDs (e.g., with features to enhance or tailor light output quality, efficiency, thermal properties, and/or controllability for a desired end use), and a need for improved devices incorporating such packages.